Method of manufacturing antimicrobial implants of polyetheretherketone

ABSTRACT

Methods of fabricating implantable medical devices, preferably with PEEK, having antimicrobial properties, are disclosed. The antimicrobial effect is produced by incorporating ceramic particles containing antimicrobial metal cations into molten PEEK resin, which is subsequently allowed to cool and set in its final shape achieved by injection molding, cutting and machining or other techniques.

This application is a continuation of U.S. patent application Ser. No. 13/512,702 filed May 30, 2012, which is a 371 of International Application No. PCT/US2010/059868 filed Dec. 10, 2010, which claims priority of U.S. Provisional Application Ser. Nos. 61/285,719 filed Dec. 11, 2009 and 61/300,629 filed Feb. 2, 2010, the disclosures of which are incorporated herein by reference.

BACKGROUND

Implantable medical devices are implanted into the body for various reasons, including orthopedics (e.g., hip replacement, spinal procedures, knee replacement, bone fracture repair, etc.). In view of the structural integrity requirements of such devices, materials of fabrication are limited, and conventionally include metal, plastic and composites.

The benefits derived from these devices are often offset by infection, which can lead to sepsis and death. The most common organisms causing infections are Staphylococcus epidermidis and Staphylococcus aureus. Other gram-positive bacteria, gram-negative bacteria and fungal organisms also are problematic. Of particular concern is Methicillin-resistant Staphylococcus aureus (MRSA), a type of staphylococcus bacteria that are resistant to many antibiotics. As a result, MRSA infections are more difficult to treat than ordinary staph infections, and have become a serious problem.

Many pathogenic bacteria can form multicellular coatings, called biofilms on bioengineered implants. Biofilms can facilitate the proliferation and transmission of microorganisms by providing a stable protective environment. These biofilms, when well developed, can disseminate bacterial planktonic showers which can result in broad systemic infection.

Bioengineered materials act as excellent hosts for the formation of bacterial biofilms. Occasionally, the implant itself carries the infecting organism, and the implants develop very tenacious biofilms seeded by infecting organisms. When this occurs, usually the implant must be removed, the patient must be treated with a prolonged course of one or more antibiotics in an effort to cure the infection, and a new implant is then re-implanted. This obviously subjects the patient to additional trauma and pain, and is extremely expensive.

Accordingly, much research has been devoted toward preventing colonization of bacterial and fungal organisms on the surfaces of orthopedic implants by the use of antimicrobial agents, such as antibiotics, bound to the surface of the materials employed in such devices. For example, silver is a powerful, natural antibiotic and preventative against infections. Acting as a catalyst, it disables the enzyme that one-cell bacteria, viruses and fungi need for their oxygen metabolism. They suffocate without corresponding harm occurring to human enzymes or parts of the human body chemistry. The result is the destruction of disease-causing organisms in the body. Silver disrupts bacteria membranes, inter-membrane enzymes, and DNA transcription.

Ceramics such as zeolite function as a cation cage, being able to be loaded with silver and other cations having antimicrobial properties. Metal zeolites can be used as an antimicrobial agent, such as by being mixed with the resins used as thermoplastic materials to make the implantable devices, or as coatings to be applied to the devices; see, for example, U.S. Pat. No. 6,582,715, the disclosure of which is hereby incorporated by reference. The antimicrobial metal zeolites can be prepared by replacing all or part of the ion-exchangeable ions in zeolite with ammonium ions and antimicrobial metal ions. Preferably, not all of the ion-exchangeable ions are replaced.

One particular thermoplastic resin that has been found to be useful in an implant is polyetheretherketone (PEEK). This thermoplastic polymer has an aromatic backbone, interconnected by ketone and ether functionality. PEEK is suitable for implants because its modulus closely matches that of bone, and is resistant to chemical and radiation damage. Grades of PEEK approved for implantation are very pure and inert and need to pass stringent cytotoxicity testing before being allowed to be implanted into mammals.

The ISO 10993 set entails a series of standards for evaluating the biocompatibility of a medical device prior to a clinical study. These documents were preceded by the Tripartite agreement and are a part of the harmonization of the safe use evaluation of medical devices. Those standards include:

-   -   ISO 10993-1:2003 Biological evaluation of medical devices Part         1: Evaluation and testing     -   ISO 10993-2:2006 Biological evaluation of medical devices Part         2: Animal welfare requirements     -   ISO 10993-3:2003 Biological evaluation of medical devices Part         3: Tests for genotoxicity, carcinogenicity and reproductive         toxicity     -   ISO 10993-4:2002/Amd 1:2006 Biological evaluation of medical         devices Part 4: Selection of tests for interactions with blood     -   ISO 10993-5:2009 Biological evaluation of medical devices Part         5: Tests for in vitro cytotoxicity     -   ISO 10993-6:2007 Biological evaluation of medical devices Part         6: Tests for local effects after implantation     -   ISO 10993-7:1995 Biological evaluation of medical devices Part         7: Ethylene oxide sterilization residuals     -   ISO 10993-8:2001 Biological evaluation of medical devices Part         8: Selection of reference materials     -   ISO 10993-9:1999 Biological evaluation of medical devices Part         9: Framework for identification and quantification of potential         degradation products     -   ISO 10993-10:2002/Amd 1:2006 Biological evaluation of medical         devices Part 10: Tests for irritation and delayed-type         hypersensitivity     -   ISO 10993-11:2006 Biological evaluation of medical devices Part         11: Tests for systemic toxicity     -   ISO 10993-12:2007 Biological evaluation of medical devices Part         12: Sample preparation and reference materials (available in         English only)     -   ISO 10993-13:1998 Biological evaluation of medical devices Part         13: Identification and quantification of degradation products         from polymeric medical devices     -   ISO 10993-14:2001 Biological evaluation of medical devices Part         14: Identification and quantification of degradation products         from ceramics     -   ISO 10993-15:2000 Biological evaluation of medical devices Part         15: Identification and quantification of degradation products         from metals and alloys     -   ISO 10993-16:1997 Biological evaluation of medical devices Part         16: Toxicokinetic study design for degradation products and         leachables     -   ISO 10993-17:2002 Biological evaluation of medical devices Part         17: Establishment of allowable limits for leachable substances     -   ISO 10993-18:2005 Biological evaluation of medical devices Part         18: Chemical characterization of materials     -   ISO/TS 10993-19:2006 Biological evaluation of medical devices         Part 19: Physio-chemical, morphological and topographical         characterization of materials     -   ISO/TS 10993-20:2006 Biological evaluation of medical devices         Part 20: Principles and methods for immunotoxicology testing of         medical devices

At high processing temperatures, metal zeolite can release moisture if it is not extremely dry. This moisture can cause the formation of voids in the polymer melt and can contribute to the decomposition of the PEEK polymer and to oxidation of metals, such as silver, copper and/or zinc, incorporated into the zeolite antimicrobial. Although the presence of voids may not be critical for certain non-load bearing applications, the absence of voids is critical for load-bearing applications such as spinal repair.

If the process of incorporating metal zeolites is carried out in air, severe oxidation can occur as the temperature is raised, and moisture and oxygen come into contact with the metal ions. Silver will rapidly darken to a dark brown or black color. Also, the incorporation of significant quantities of metal zeolites into the PEEK polymer can affect the viscosity and rheology of the composition.

The present disclosure is based on the finding that it is possible, under conditions of high temperature and high shear, to incorporate antimicrobial zeolite, such as silver zeolite, into PEEK, such as by mixing doped metal zeolites into molten PEEK (melting point between 300 and 400° C., depending on purity), followed by molding and processing of the composite blend. The result is the provision of medical devices such as implants with effective antimicrobial activity in order to reduce the growth of bacteria and risk of infection.

SUMMARY

The shortcomings of the prior art have been overcome by the present disclosure, which relates to methods of fabricating implantable medical devices, preferably with PEEK, having antimicrobial properties. The antimicrobial effect is produced by incorporating ceramic particles containing antimicrobial metal cations into molten PEEK resin, which is subsequently allowed to cool and set in its final shape achieved by injection molding, cutting and machining or other techniques.

The rate of release of metallic ions is governed by the extent of loading of the plastic with ceramic particles containing metal ions and the loading of metal in the ceramic material. The electrolyte concentration in blood and body fluids is relatively constant and will cause ion exchange with ions such as silver, copper and zinc, and others from the surface of the implant, which deactivate or kill gram positive and gram negative organisms, including E. coli and Staphylococcus aureus. Effective antimicrobial control of a six-log reduction of microorganisms is achieved, for example, at metal zeolite concentrations of 4% or greater. The devices are implantable into animals, particularly humans. Spinal implants are particularly contemplated.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a perspective view of a cervical spacer in accordance with certain embodiments.

DETAILED DESCRIPTION

Embodiments disclosed herein relate to the processing of ceramics, preferably zeolites, as a cation cage in combination with medical implants to deliver and dose one or more antimicrobial cations. In addition to zeolite, other suitable ceramic antimicrobial materials include hydroxy apatite, zirconium phosphates and other ion-exchange ceramics.

Suitable cations include silver, copper, zinc, mercury, tin, lead, gold, bismuth, cadmium, chromium and thallium ions, and combinations thereof, with silver, zinc and/or copper being preferred. Either natural zeolites or synthetic zeolites can be used to make the zeolites used in the embodiments disclosed herein.

“Zeolite” is an aluminosilicate having a three dimensional skeletal structure that is represented by the formula: XM_(2/n)O.Al₂O₃.YsiO₂.ZH₂O wherein M represents an ion-exchangeable ion, generally a monovalent or divalent metal ion, n represents the atomic valency of the (metal) ion, X and Y represent coefficients of metal oxide and silica respectively, and Z represents the number of water of crystallization. Examples of such zeolites include A-type zeolites, X-type zeolites, Y-type zeolites, T-type zeolites, high-silica zeolites, sodalite, and others.

In addition, silicate materials such as metal doped phosphate glass, bioactive glass such as 45S5 and BG can be processed to deliver a suitable dose of antimicrobial cations.

The following process and handling conditions are required for processing antimicrobial PEEK implants with the desired properties.

Material Handling

PEEK is a high melting point material that has a melting point of approximately 340° C. and must be processed at 360° C. to 400° C. to enable incorporation of metal zeolite powders and extruding or molding of the composite blend. At these high temperatures, release of any entrapped moisture can cause the formation of voids in the polymer melt. This can contribute to decomposition of the PEEK polymer and to oxidation of metals incorporated into the zeolite antimicrobial, reducing antimicrobial efficacy. Additionally, presence of voids can significantly degrade the properties of the final product. For material produced for load-bearing spinal implants, the minimization of the size and number of any voids is critical in maintaining the mechanical properties of the final product. Therefore, it is essential to remove residual moisture from the raw materials utilized in producing the implant, PEEK and antimicrobial powder (composite blend components). The silver zeolite powder should be heated to about 400° C. at atmospheric pressure, and held there for sufficient time for the bound water to be released. If the powder is spread thinly, moisture release will be more efficient, and a holding time of 30 to 60 minutes is suitable. The silver zeolite can also be dried at lower temperatures under reduced pressure. It is preferable that the composite blend components be dried in a clean environment to less than 0.1% moisture by weight prior to processing. Moreover, it is imperative to maintain the material in an anhydrous environment prior to and during processing.

The antimicrobial powder is extremely hygroscopic, requiring the material to be sealed in its original packaging prior to any usage. Exposure to ambient air has to be minimal during pre-blending operations. As an added precaution, the feeder powder hopper needs to be purged with dry nitrogen immediately before and after loading.

The PEEK material should be dried between 120° C.-130° C. for 12 hours (or at a time/temperature equivalent) before running any compounding operation. This ensures that the residual moisture of the PEEK pellets is less than 0.1% moisture by weight prior to processing.

Post-Processed Packaging: The materials need to be packaged in moisture barrier bags immediately after compounding.

Compounding Equipment

To achieve a high degree of consistency, a twin-screw extruder can be utilized for blending PEEK with required additives. The additive is gravimetrically metered into molten PEEK just prior to entering the extruder screws to obtain the desire additive loading. The use of twin-screw extrusion improves filler distribution and wet-out, and results in a more reproducible rheological behavior.

A specific example of preferred equipment used for compounding is described below:

-   Extruder Line: 30 mm -   Screw design: 30-3 -   Die set-up: 2 hole -   Acceptable Range of Pellet Length (inches): 0.100-0.135 -   Acceptable Range of Pellet Diameter (inches): 0.085-0.120

One suitable extruder is a Leistritz ZSE twin screw extruder.

Compression Molding

Near-net shaped compounded PEEK blanks suitable for secondary machining operations can be produced using the compression molding process. Compression molding is a method of molding preheated compounded PEEK polymer pellets or pre-forms into predefined shapes in pressurized, heated mold cavities. Compression molding is a high-volume, high-pressure method suitable for molding complex shapes. This process wastes relatively little material when compared to machining devices from extruded rods.

Machining

PEEK implants can be manufactured to final shape by removing material with machining process such as turning, boring, drilling, milling, broaching, sawing, shaping, planing and/or reaming. These machining processes can be manual or automated through the use of computer controlled machining centers.

The thermal conductivity of all polymeric materials is lower than that of metals, so heat build-up during machining is rapid. A cleaner cut and more open surface can be achieved if the bit is cooled and run at a slower rate. Machined PEEK surfaces can smear and “skin” if the PEEK surface temperature significantly rises while the material is being machined into implants, possibly affecting the release of cations. To optimize the compounded PEEK surface finish, a clean cut and “open” surface structure is desired. One method to achieve the desired surface finish is to cool the cutting tool with cold, clean compressed air in conjunction with optimized tool cutting speed and feed rates. Moreover, cooling rates can have a profound effect on the crystallinity of PEEK, which may be critical to optimize the rate of release of metallic ions. By adjusting the cooling rate during fabrication, the percent crystallinity of the implant material was carefully controlled.

Machining and finishing operations on PEEK materials are prone to propagating residual stresses. Before machining, it is recommended that components formed from PEEK be annealed to relieve stress. During machining or finishing, further stresses may be built-up within the material by localized heating at the cutting point. Therefore, if a large amount of machining and finishing is to be carried out on a component, a second annealing procedure could be required. Based on the desired outcome of the annealing process (removal of stresses or thermal history or the optimization of crystalline structure) annealing processes recommended by the PEEK material manufacturer should be requested and followed.

Additional precautions can be taken to prevent surface contamination of machined compounded PEEK devices by dedicating machine tools as “PEEK only” and locating the machine tools in areas of the manufacturing facility specifically dedicated to the machining of medical grade PEEK.

Injection Molding

Finished or significantly finished compounded PEEK devices can be manufactured using the injection molding process. The injection molding process produces parts from compounded PEEK polymers by feeding compounded PEEK pellets into a heated barrel (400° C.) where the molten compounded PEEK is mixed and forced into a heated mold cavity that is maintained at temperatures between about 175° C. and 205° C. Once injected into to the mold, the molten compounded PEEK cools to a temperature below 343° C. and hardens to the shape of the mold cavity. By monitoring and closely controlling the desired temperature (between about 175° C. to 205° C.) set-point of the molds, significant increases in part feature and tolerance control and minimization of surface skinning can be achieved.

Optimal Zeolite Loading

The amount of metal zeolite incorporated into the resin should also be an amount effective for promoting antimicrobial activity; e.g., a sufficient amount so as to prevent or inhibit the growth and eradicate bacterial and fungal organisms. At the same time incorporation of significant quantities of metal zeolites into the peek polymer melt can affect the viscosity and rheology of the composition. Therefore, a processing window was established that allowed for sufficient zeolite to be loaded without adversely affecting the bulk properties of the final product.

Suitable amounts of zeolite in the resin range from about 0.01 to 20.0 wt. %. Optimal loading was found to range from 0.1 to about 10.0 wt. %. The tightly controlled and monitored dimensions of the pellets produced from the optimal loading conditions are shown below in Table I. The finished pellet color is an important indicator if severe oxidation occurred or moisture came into contact with the metal ions during the high temperature process. Pure PEEK is light tan in color while properly processed zeolite containing PEEK is brown in color. In cases where oxidation has occurred, the silver will rapidly darken to a dark brown or black color.

TABLE I Average pellet length and diameter Average Pellet Average Pellet Zeolite Loading Diameter(inch) Length (inch) Color 0.5% 0.090 0.126 Brown 1.0% 0.093 0.127 Brown 2.0% 0.093 0.127 Brown 4.0% 0.091 0.125 Brown Confirmation of Embedded Zeolites

Scanning electron micrographs (SEM) of the samples revealed that dispersion of the zeolite particles in the composites were satisfactorily uniform. The SEM images show that the samples are highly loaded, consistent with the target loading rates.

Pyrolysis was utilized for quantitative determination of the loading of silver metal zeolite. A small accurately weighed sample of the PEEK/silver zeolite composite was placed in a ceramic crucible and burned with a propane torch. While plain PEEK burns away completely leaving no residue using this method, PEEK loaded with metal zeolites will burn off and leave behind a powdery residue. The amount of silver zeolite loading can then be determined gravimetrically. The amount of silver present can be confirmed by extracting the silver from the residue and determining the amount of silver in the extraction solution by graphite furnace AA or ICP.

Quantification of Eluted Silver

The elutable ionic silver content from samples with different loading rates was determined by chemical analysis using graphite furnace atomic absorption spectroscopy. 1″×1″ sample coupons were immersed into 40 ml of 0.8% sodium nitrate solution for a duration of 24 hours. The amount of silver eluted from different loadings is listed in Table II.

TABLE II Amount of silver eluted from different loading conditions Eluted Silver Concentration (ug/L) Zeolite Loading Run 1 Run 2 Run 3 0.5% 1 <1 <1 1.0% <1 <1 <1 2.0% 1.5 2.1 1.3 4.0% 4.8 10 4.7 Elution of other antimicrobial metals can be determined similarly. Antimicrobial Efficacy

It is not self-evident that silver or other metal zeolite incorporated into molten PEEK will show adequate antimicrobial control when the surface of the material is exposed to a microbial challenge. Bacterial challenge tests were performed with a gram negative (E. Coli) and a gram positive (Staphylococcus aureus) microorganism with two different loadings (2% and 4%) being tested. The results show that for both organisms, a 4% loading was effective in controlling the colony forming units (CFU) of bacteria after an exposure of 24 hours. As Table III highlights, control samples without any zeolite showed no bacterial reduction, while the 4% loadings showed at least a six log reduction of microorganism over the stated period of time.

TABLE III Antimicrobial Efficacy of against a gram negative and a gram positive organism Zeolite Loading Test Organism Average Reduction in CFU 0% E. Coli No Reduction (Control Sample) 2% E. Coli   77.0% 4% E. Coli 99.99992% 0% S. Aureus No Reduction (Control Sample) 2% S. Aureus   30.5% 4% S. Aureus 99.99998% Radiopacity

One disadvantage of PEEK implants is that because of PEEK's radiolucent properties implanted devices made from PEEK do not show up well in X-ray images. Thus, identification of the precise location and integrity of the implant and observation of other important characteristics from X-rays can be difficult. One method that has been used to overcome this drawback is the addition of barium sulfate to the compounding mix. Although devices can be radiopaque due to the addition of barium sulfate to PEEK, the addition of the required amounts of barium sulfate for the desired radiopacity tends to weaken the strength of the resulting device. Thus, the use of an implant made from a composition of barium sulfate and PEEK provides an undesirable tradeoff between the desired mechanical properties of the device and its radiopacity. The materials disclosed herein display radiopaque properties even at low loading levels of silver zeolite when viewed under x-ray. This is an important additional benefit that can be derived without sacrificing antimicrobial effectiveness or mechanical integrity of the material.

Enhanced Cell Attachment

Zeolite at the surface of the implant can significantly increase osteoblast cell attachment due to the presence of negatively charged silicates present in the zeolites. A significant drawback of PEEK products oftentimes is the lack of cell adhesion at the implant site due to the inherent inertness of PEEK. This problem can be suitably alleviated through the availability of by negatively charged silicates at the implant surface. The zeolite structure is inherently negatively charged. These silicates attract proteins containing the RGD peptide sequence with the correct confirmation, leading to the attachment and proliferation of bone forming osteoblast cells. This starts a cascading process that eventually results in significant bone growth.

Surface Roughness Optimization

PEEK is very impermeable to moisture so only material in the surface layer no deeper than the diameter of a zeolite particle will be accessible for silver elution. If the silver in this region were deactivated or otherwise converted into insoluble complexes the materials might well be ineffective as antimicrobial materials. So process conditions need to be carefully monitored and followed to maintain silver effectiveness on the implant surface. Effectiveness can be monitored quantitatively by measuring the elutable silver using an ICP and ensuring that the level is well above the reported minimum biofilm growth inhibitory concentration of 0.1 ppm of silver. 20 ppb silver has been shown to be active in the lab and formulations show antimicrobial activity below 5 ppb. Moreover, while the bulk solution concentration may be 5 ppb, the activity of cations close to the surface must necessarily be better.

Surface color can also be used as a qualitative measure for monitoring process conditions. Deactivated or otherwise converted silver will change color from brown to black or a mixture of black-brown.

Additionally, surface roughness can be optimized to improve osteogenic properties of the implant. In-vitro studies have shown that all parameters (adhesion, proliferation, alkaline phosphatase activity, synthesis of bone matrix proteins and mineralization) effecting bone growth appear to be influenced by the surface finish of the material. Common manufacturing techniques such as abrasive blasting (bead blasting, sandblasting, sodablasting, cryogenic CO₂ blasting etc.), scraping or sanding with inert materials can be utilized to control the surface roughness to optimize both silver elution and cell growth. Atomic force microscopy (AFM) or Profilometry can be utilized to calculate the average surface roughness in nanometers and this value can be correlated to bone growth parameters from vitro cell testing results to get the optimum range of surface roughness.

Increase in Vascularization

Early lumbar and cervical spacers had a monolithic construction in the majority of the cases. The goal was to create polymeric mimics of allograft cortical bones, which had been the previous gold standard for fusion. The current evolution of implant design has been toward openings between inner and outer surfaces for vascularization of the new bone mass. The cavity of the spacer is important as it receives outside material (fillers) and may be a carrier of pathogens. The columns of the implants disclosed herein has been designed so that it allows for a mechanically robust implant, while increasing vascularization pathways, and making large areas available for silver elution.

A specific example of the design is shown in FIG. 1. As can be seen from the FIGURE, the columnar design allows for a significant increase in surface area compared to a monolithic construction. This leads to a significantly enhanced silver elution profile as well as increasing the pathways for vascularization.

Adding Porosity

If desired, in certain embodiments the PEEK can be made porous with suitable porosities including porosities between 50% and 85% by volume. Porosity can be imparted using a pore forming agent such as sodium chloride, to create a porous polymer comprising a plurality of interconnected pores, by processes known in the art. Average pore size is generally greater than 180 microns in diameter, suitably between about 300 and about 700 microns.

Resin Reinforcement

If desired, in certain embodiments the PEEK can be reinforced with a reinforcing material such as ceramic or carbon fiber. This can be produced by dispersing the reinforcing material in the PEEK polymer matrix, such as by twin screw compounding of implantable PEEK polymer with carbon fibers. The resulting carbon fiber-reinforced product can be used to direct injection mold final devices and near net shapes, or it can be extruded into stock shapes for machining. The incorporation of fibers or other suitable reinforcing material(s) provides high wear resistance, a Young's modulus of 12 GPa (matching the modulus of cortical bone) and providing sufficient strength to permit its use in very thin implant designs which distribute the stress more efficiently to the bone. The amount of reinforcing material such as carbon fiber incorporated into the PEEK can be varied, such as to modify the Young's modulus and flexural strength. One suitable amount is 30 wt. % carbon fiber. 

What is claimed is:
 1. A method of making a load bearing antimicrobial implant, comprising drying polyetheretherketone resin to a residual moisture content of less than 0.1% moisture by weight, heating said dried polyetheretherketone resin to a temperature effective to melt said resin, blending into said molten resin a metal zeolite comprising silver zeolite, cooling said blend, and forming said blend into said implant.
 2. The method of claim 1, comprising drying said resin by heating to a temperature between about 120° C. and 130° C.
 3. The method of claim 1, comprising drying said zeolite at a temperature of about 400° C. to remove residual moisture prior to said blending step.
 4. The method of claim 1, wherein said polyetheretherketone is reinforced with carbon fiber.
 5. The method of claim 1, wherein said blending step is carried out in a twin screw extruder.
 6. The method of claim 1, wherein the amount of zeolite blended into said resin is from about 0.01 to about 20.0 wt %.
 7. The method of claim 1, wherein said polyetheretherketone is porous.
 8. The method of claim 1, wherein said polyetheretherketone has a porosity between 50% and 85% by volume. 